In magnetic resonance imaging (MRI), outer volume suppression (OVS) is a technique designed to reduce signal outside a region of interest (ROI) to facilitate faster acquisitions and improve image quality within the ROI. By creating an effectively smaller region to image, the field of view (FOV) can be reduced or restricted to reduce scan time or facilitate higher resolution imaging. Additionally, OVS can improve image quality by suppressing moving anatomy and thus the motion artifacts that may otherwise arise. Suppressing signal outside the ROI also improves the dynamic range inside the ROI.
Previous techniques for OVS have been applied to spectroscopy, diffusion, functional MRI (fMRT), and cardiac imaging. The methods largely fall into two paradigms. One paradigm utilizes sequences that interleave and arrange multiple 1D slab saturation pulses. In spectroscopy, there are several techniques that arrange the saturated slabs in a polygon to primarily suppress subcutaneous fat near the skull. BISTRO is an adiabatic pulse for general applications that similarly arranges saturation slabs in a polygon to achieve 2D OVS. In spinal diffusion, cardiac, and black-blood angiography, there are OVS techniques that saturate slabs on two sides of the ROI to reduce FOV in the phase-encode direction. Also, a projection reconstruction (PR) technique that also saturates slabs on two sides but rotates the saturation direction with the PR acquisition direction to effectively create a 2D OVS.
The other paradigm for OVS is to utilize one spatially selective pulse and one spatially non-selective pulse for a tip-down and tip-up combination. For non-selective tip-downs, BIR-4 pulses are used for their adiabaticity and broad bandwidth. For non-selective tip-ups, adiabatic half passage or sinc pulses are used. Recent techniques of this paradigm utilize a 2D spiral pulse for spatial selectivity as either a tip-down or tip-up pulse. Sequences for general applications play the tip-down pulse and tip-up pulse with minimal time in between. For cardiac imaging, T2-preparation is beneficial and these sequences incorporate double adiabatic full passage (AFP) refocusing.
Current OVS techniques may have long durations, high specific absorption rate (SAR), or an ROI with inflexible selectivity. Thus, there is a need for improvements in OVS techniques.